An internal combustion engine may couple to an exhaust aftertreatment system to purify an exhaust gas feedstream. An exemplary exhaust aftertreatment system may include purification devices such as oxidation catalysts, reduction catalysts and particulate filters. An exemplary exhaust aftertreatment system may include monitoring devices such as temperature sensors and exhaust gas constituent sensors. An engine controller may employ feedback from the monitoring devices to control operation of the internal combustion engine and to monitor operation of the internal combustion engine. One exhaust gas constituent sensor monitors nitrogen oxides (NOx) from the engine.